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Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (Ariel) has been adopted as ESA “Cosmic Vision” M4 mission, with launch scheduled for 2029. Ariel is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 μm, operating in cryogenic conditions in the range 40–50 K. Aluminum has been chosen as baseline material for the telescope mirrors substrate, with a metallic coating to enhance reflectivity and protect from oxidation and corrosion. As part of Phase B1, leading to SRR and eventually mission adoption, a protected silver coating with space heritage has been selected and will undergo a qualification process. A fundamental part of this process is assuring the integrity of the coating layer and performance compliance in terms of reflectivity at the telescope operating temperature. To this purpose, a set of flat sample disks have been cut and polished from the same baseline aluminum alloy as the telescope mirror substrates, and the selected protected silver coating has been applied to them by magnetron sputtering. The disks have then been subjected to a series of cryogenic temperature cycles to assess coating performance stability. This study presents the results of visual inspection, reflectivity measurements and atomic force microscopy (AFM) on the sample disks before and after the cryogenic cycles.

Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (ARIEL) is the M4 ESA mission to launch in 2028. ARIEL is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 μm and 7.8 μm (main instrument), operating in cryogenic conditions in the range 50 - 60 K. For the main mirror substrate, the Aluminum 6061 alloy has been chosen as baseline material after a trade- off. The large size of the mirror however (0.6 square meters) presents specific production challenges concerning opto-mechanical stability in cryogenic applications. To minimize risk, the machining, polishing, thermal treatments and coating processes will first be tested on flat samples of 150 mm of diameter and then applied to a full-size demonstrator mirror, before finalizing the design and producing the flight mirror. This study, following a review of existing literature on fabrication of Al 6061 mirrors for spaceborne IR applications will characterize the optical properties of the samples after each phase of thermal treatment with the goal of determining an optimal process for material stress release, figuring and surface finishing and final optical stability in the operating cryogenic environment.

The increasing number of Earth Observation missions launched over the last decade has stimulated the development of a large number of satellite instruments able to acquire and deliver rich imageries suitable to support many different applications. Recent advances in electronics, optical manufacturing and remote sensing are now enabling the conception of smaller instruments that could enable new mission concepts at lower costs such as the adoption of satellite constellations for improved temporal resolution. In this paper we present the development of an innovative optical payload named STREEGO suitable for Earth Observation from Low Earth Orbit (LEO) microsatellites. STREEGO is an athermal, fully reflective telescope based on a three mirror anastigmat (TMA) design which features a 200 mm aperture, a focal length of 1.2 m and an across-track Field of View (FoV) of about 2°. Leveraging on a large format two-dimensional CMOS sensor with a pixel size of 5.5 μm, it delivers a nominal modulation transfer function (MTF) of 64% at Nyquist frequency and a ground sampling distance of 2.75 m from an altitude of 600 km. In the design of the instrument detailed stray-light and tolerance analyses were performed and a worst-case thermal model was also developed to ensure that optimal image quality is achieved under operational conditions. After preliminary tests on a Demonstrator Model (DM), an Engineering Model (EM) of the payload with a mass of 20 kg including its electronics and mounting interfaces has been integrated and tested in laboratory and it is now ready to start an environmental test campaign to increase its Technology Readiness Level (TRL). The qualification of the instrument and the results achieved are presented in detail.